1. Field of the Invention
This invention relates to a method for removing metal impurities from resist components. In particular, this invention relates to a method for removing metal impurities (including sodium, iron, calcium, chromium, copper, nickel, and zinc) from a resist component or resist composition solution by contacting that solution with a cation exchange resin which has been prewashed with a quaternary ammonium salt solution.
2. Brief Description of Prior Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components such as in the fabrication of integrated circuits and printed wiring board circuitry. Generally, in these processes, a thin coating or film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or copper plates of printed wiring boards. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam, and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.
There are two types of photoresist compositions--negative-working and positive-working. Both negative-working and positive-working compositions are generally made up of a film-forming resin and a photoactive compound dissolved in a suitable casting solvent. Additives may be added for specific functions. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation becomes less soluble to a developer solution (e.g., a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to a developing solution. Thus, treatment of an exposed negative-working resist with a developer solution causes removal of the nonexposed areas of the resist coating and the creation of a negative image in the photoresist coating; and thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the resist composition exposed to the radiation become more soluble to the developer solution (e.g., a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working resist with the developer solution causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
After this development operation, the now partially unprotected substrate may be treated with a substrate etchant solution, plasma gases, or the like. The etchant solution or plasma gases etch the portion of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected and, thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining resist layer after the development step and before the etching step to increase its adhesion to the underlying substrate and its resistance to etching solutions.
Positive-working photoresist compositions are currently favored over negative-working resists because the former generally have better resolution capabilities and pattern transfer characteristics. Preferred positive-working photoresist today generally involve novolak resins and o-quinonediazide photoactive compounds dissolved in a suitable solvent.
Impurity levels in photoresist compositions are becoming an increasingly important concern. Impurity contamination, especially by metals, of photoresists may cause deterioration of the semiconductor devices made with said photoresists, thus shortening these devices' lives.
Impurity levels in photoresist compositions have been and are currently controlled by (1) choosing materials for photoresist composition which meet strict impurity level specifications and (2) carefully controlling the photoresist formulation and processing parameters to avoid the introduction of impurities into the photoresist composition. As photoresist applications become more advanced, tighter impurity specifications must be made.
In the case of novolak resin materials used for making positive photoresists, such novolak resins have been subjected to distillation or crystallization purification operations in order to remove impurities, especially metals. However, such operations have deficiencies. One, they are time-consuming and costly. More importantly, they do not remove impurities down to the very low levels now needed for advanced applications (i.e, in low parts per billion maximum levels).
Alternatively, ion exchange resins have been used for novolak impurities. One general technique is to pass an impure novolak resin solution through a cation exchange resin (e.g., AMBERLYST styrene sulfonic acid-divinyl benzene cation exchange resin). However, such treatments have several problems associated with it including the following:
1. The cation exchange resin treatment of the novolak may decrease the pH of the novolak-containing solution, possibly causing serious corrosion of metal containers in which the purified novolak-containing solution may be stored.
2. The purified novolak may have a decreased rate of dissolution during the development step of the photoresist which may be caused by the undesired adsorption of the lower molecular weight portion of novolak resin onto the cation exchange resin.
3. Alkali metals such as sodium and potassium are easily removed with conventional cation exchange resins. However, divalent or trivalent metal cations (e.g., Cu.sup.+2, Ni.sup.+2, Zn.sup.+2, Fe.sup.+2, Fe.sup.+3, Ca.sup.+2, or Cr.sup.+3 ions) may have a lower affinity to conventional cation exchange resins. Iron and other easily oxidizable metals cannot be completely removed because they may be colloidal metal hydroxides or oxides. Such colloidals are not significantly removed by cation exchange resin treatment.
An ion exchange resin, particularly a strong acid-type of cation exchange resin, may decompose hydrolyzable resist components or solvents, therefor, such as ester groups. For example, ethryl lactate is decomposed by AMBERLYST A-15 to form polylactide moieties, which may degrade lithography performance of photoresists. As used herein, the term "polylactide" is defined as a polymeric or oligomeric product of lactide, a cyclic dimer of lactic acid which is formed by hydrolysis of ethyl lactate.
In addition to the standard cation exchange resin treatment of the novolak resin, it is known to subject complete photoresist compositions (e.g., novolak resin, photosensitizer, and solvent) to both cation and anionic exchange resin treatment. For example, Japanese Patent Publication (Kokai) No. 57-74370 discloses a method of reducing impurities in resists by using cation exchange resins and anion exchange resins in separate and a successive manner. Japanese Patent Publication (Kokai) No. 01-228,560, which was published on Sep. 12, 1989, teaches that the metal impurities content in photosensitive resin solutions or photoresist compositions may be reduced with a mixture of a cation and anion exchange resins. However, these techniques have the deficiency of not removing divalent and trivalent metal impurities and may decompose resist components or solvents containing resist components.
Usually, such cation and anion exchange resins have been washed with a solvent such as deionized water or the same solvent used to dissolve the resist component is already dissolved in. However, such washings with water or solvents will not clean the resins of pre-attached metal impurities because metal ions such as sodium or potassium as well as other acidic contaminates strongly bond to the anionically charged groups of cation exchange resins.
Accordingly, there is still a need in the photoresist art for improved methods of removing metal impurities from novolak resins and other materials used as photoresist components. The present invention is a solution to that need.